Treatment of woody material



Jan. 14, 1941. s. FELDMAN ErAL TREATMENT OF WOODY MATERIAL Filed NOV.28, 1956 2 Sheets-Sheet 1 naam uvas/MQW was uvas; 4 mwkm.

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TREATMENT OF WOODY MATERIAL Filed NOV. 28, 1956 2 Sheets-Sheet 2 .www

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Patented Jan. 14, l94l 'UNITED STATES TREATMENT F WOODY MATERIAL SoldonFeldman, East Alton, Ill., and Fredrich Olsen, St. Louis, Mo., assignorsto The Cellulose Research Corporation, East Alton, Ill., a corporationof Delaware Application November 28, 1936, Serial No. 113,124

Claims.

4This invention relates to the impregnation of Wood and woody materialand more particularly to the impregnation and pulpingof wood withalkaline reagents.

A primary object of the invention is to provide a process for uniformlyimpregnating wood and woody materials with liquids, chemical reagents,dyestuffs, preservatives and the like.

Another object isrto provide a process for the treatment of wood whichincludes a means for facilitating the uniform distribution of analkaline reagent throughout the wood subdivisions prior to theattainment of temperatures at which substantial rates of delignicationof the wood are obtained.

A. further object is to provide a process whereby the substantiallyuniform removal of any desired proportion of non-cellulosic substancesfrom the raw material can be accomplished by digestion with alkalineliquors.

A still further object is to provide a process for the alkaline pulpingof wood whereby a given extent of deligniflcation can be nobtained in a.shorter digestion period and'with a lower consumption of chemical thanis demanded by the prior practices.

A further object of the invention is to-provide means for facilitatingthe uniform distribution throughout the woody material of suchpredetermined amount of reagent as is required for the treatment of itsnon-cellulosic components so that on attainment of reactive temperaturesthe necessity for the transfer of chemical from the liquor surroundingthe wood subdivisions will be obviated.

A further object is to provide a process for treating wood whichminimizes the conditions which normally hinder or prevent uniformimpregnation with treating solutions and more especially, thel uniformdigestion of reagents, pre.

servatives, dyestus, or other substances throughout the wood and eachsubdivision thereof.

The natural morphological structure of wood presents serious ldiilicultyto its uniform penetration by liquids. It comprises a highly complexstructure -of capillary passage and interstices which are normallyfilled with air and encrusting substances in a variety of forms, rangingfrom more or less freely flowing liquid to a state of semi-immobility,or even complete immobility. VThis condition renders it quite difficultto impregnate and evenly distribute liquids throughout the subdivisionsof the wood by the usual methods. In addition, the wood preferentiallyadsorbs certain reagents, preservatives. dyestuffs,

and other substances held in solutions. Consequently, in many cases,even where it may be possible to effect impregnation of the solutions,the reagents and other substances which it is desired to uniformlydistribute may be largely deposited on the exterior portions of thesubdivisions;

. therefore, little, if any, of said substances reach special attentionto the subsequent time of im.

pregnation with a delignifying solution.

Figure #3 is a graph illustrating the effect of a pretreatment of thepresent invention as com-f pared to certain prior practices.

Figure #4 is a diagrammatical representation of capillary tubescontaining air under conditions of freely vented boiling.

Figure #5 is a diagrammatical sketch similar to Figure #4 illustratingthe action of air in similar tubes where the heating is under pressureconditions.

It has long been known that the conventional processes for reducing woodto cellulosic pulp inwhich the Wood subdivisions are charged into adigester, delignifying solutions added and heating of the mass started,are far from ideal since the delignification reaction starts on theouter exposed surfaces of the wood subdivisions and gradually proceedsto the inner regions.V Since the delignifying reactions under suchconditions are relatively slow, the outer portions of the wood subjectedto the chemical action of the treating liquors for increasingly longerperiods of time than other areas progressively distant from the outersurfaces of the subdivisions. Furthermore,

the concentration of the delignifying reagents` progressively decreasesduring vthe course of the reaction so that when the delignifying liquorsnally reach the innermost portions of the wood subdivisions theconcentration of the reactive vsubdivisions are the first to bedelignifed and arey chemicals in the solution has been greatly, de-

ple'ted.

These unfavorable conditions provided by the prior processes are furtheraccentuated by the tendency of the Wood to preferentially adsorb thereagent from some of thecommonly used treating solutions, such as. forexample. sodium hydroxide solutions. .In such cases, a large excess ofreagent, for example, of the total reagent, will be accumulated on theexteriorportions of the wood while the interior portions will have aninsufficient amount, for instance, 10% of the total reagent or even noreagent at all.

Still another factor contributing to the unequal distribution of reagentis the pronounced swelling eifect of some of the usual treatingsolutions, such as sodium hydroxide and kraft liq- When the chips comein contact with such solutions, the swelling of the exterior portionsthereof tend to constrict or even close the capillary passages andchannels existing in the woody structure, so that penetration of liquidsinto the interior portions becomes even more difficult. Furthermore, thedelignifying action of the treating liquors itself tends to obstructpenetration by producing colloidal substances capable of mechanicallyblocking or plugging the membranes or capillaries and interstices of thewood through which the liquors must penetrate.

The combined effect of such adverse conditions is to reduce the yieldand quality of pulp since the portions of the pulp delignied rst are incontact with the delignifying solutions at reactive temperatures forconsiderable periods of time. It is a well known fact that cellulosicmaterials are rapidly degraded and pass into solution under suchconditions. Furthermore, since various portions of the pulp are exposedto this action for widely varying lengths of time, a predominatingcharacteristic of the resulting pulp product is non-uniformity.

The ideal condition and one of the important features of this inventionis uniform and intimate association of all parts of each woodsubdivision with the required amount of delignifying reagent beforereactive conditions are attained, so that the reaction may proceeduniformly. rapidly, and simultaneously throughout the mass of woodsubdivisions under treatment. These conditions are provided by treatingthe wood subdivisions in a boiling solution of a non-delignifyingelectrolyte of very dilute and critical concentration under freelyvented conditions for about 30 minutes, or until all of the air withinthe capillaries and other interstices of the wood subdivisions has beenremoved. During the boiling period the chips are maintained submergedand upon the termination of the boiling, the temperature is allowed todrop a few degrees, the chips still being maintained submerged, with theresult `that the condensable vapors within the chips are condensed. Thiscreates a partial vacuum within the wood which causes the surroundingnon-delignifying electrolyte`solution to satuf rate the wood. Thetreated chips are maintained submerged in the solution for approximately30 minutes after the termination of the boiling. The excess water isthen drained from the chips, care being taken to maintain them saturatedto prevent the reentry of any air and a delignlfying solution, forexample, sodium hydroxide. at a temperature preferably just under andnot exceeding the order of C., is added. The caustic reagent becomesuniformly distributed throughout the wood and the wood is then in anideal condition for pulping.

.An illustration of the concentration of the electrolyte solutions whichwe may employ is 1x10--5 molar for aluminum sulfate and 1x10-2 Imolarfor sodium chloride, both being the types of electrolytes satisfactoryfor the purpose.

Accordingly, ,Z the process of this invention as applied to the pulpingof wood provides for the uniform distribution of delgnifying reagentsthrough the Wood prior to attaining active digestion conditions byremoving substantially all of the air from the wood and thereafterpretreating it with a non-delignifying electrolyte solution Within acritical range of concentration and subsequently treating the wood witha delignifying reagent.

By this process, saturation of the wood with digestion liquors isattained under conditions which insure uniform distribution of therequired amount of treating reagent throughout each subdivision of thewood at temperatures not greater than 100 C. This permits the adjustmentof the pulping conditions to a lenient character which results in theproduction of higher yields of pulp of exceptional uniformity and higherquality. A characteristic feature of the pulp products resulting fromthe process is their substantially undegraded cellulosic content.

These enhanced results are due primarily to the following factors:

A more rapid rise and greater uniformity of the temperature within thechips is attained due to the marked increased thermal conductivity overunimpregnated chips.

'The uniform availability of the required amount of chemical pe mits thechemical action with the encrustants ,f the wood to proceed at themaximum rate, simultaneously in all portions of the Wood subdivisions.

The wood subdivisions may be provided with just the requisite amount andno more of delignication reagent to remove the desired ,amount ofligneous material and then removed from the presence of additionaldelignication reagent so that continued or excessive degrading action'of the delignifying reagent on the cellulose after the desired amount ofdelignication is accomplished can not occur.

The above factors permit less drastic cooking conditions, i.V e., lower.chemical to Wood ratios and decreased time and temperature of digestionwith consequent less chemical action on the ce1- lulose itself than canbe obtained in the prior practices.

Saturation of the wood with the treating liquor can be accomplished onlyafter the thorough removal of the air and other gases which normallyoccupy a large portion (50%80%) of the volume of Wood as usuallyprepared for pulping. An important feature and one of the objects ofthis processcis the substantially complete removal of air from the chipsby submerging them in an aqueous, non-delignifying chemical solutionwhich is maintained in a state of active ebullition. A boiling period of15-30 minutes is sumcient to effect air removal in the case of mostdeciduous woods, for example, yellow birch, aspen, soft maple and blackgum, which constitute the usual raw materials for so-called soda pulps.Slightly longer boiling periods are required for the coniferous woodscommonly used for kraft and sulflte pulps such as spruce, the hemiocksand pines, and for woods which contain a high percentage of heartwood.

An explanation of this method of air removal from the wood subdivisionsis that under conditions of active ebullition in the surrounding liquor,the total press'ure within the capillaries of the Wood, i. e., the sumof the partial pressure of air and water vapor is greater than theexternal pressure as long as any air is present and water vapor and airescape from within the chips because oi the existence of this pressuredinerential, the air being carried off in the current of steamgenerated. The presence of liquid in sures the rapid and uniformdistribution of heat to the chips during the boiling process and makespossible the efficient and complete removal of air. This result can notbe obtained by the use of steaml alone or the use of pressure, vacuumand `other means as has been suggested in prior pracvfood with liquidcan be edected by cooling the mass slightly below the boiling point,followed by a soaking period of l hour or less. When the temperature oithe chips is thus reduced slightly below atmospheric boiling, the Watervapor with in the wood chips condenses and the capillaries become filledwith liquid which is drawn into the wood from the surrounding supply.The thor- @ugliness of air removal by this procedure is illustrated bythe following percentages oi weight increase on the dry weight oi woodon immersing` chips in boiling water for :lil minutes and thereaftersoaking at 30 C. for 60 minutes: yellow birch, and a. mixture of hemlockand balsam, 255%. The corresponding values at saturation obtained afterextended boiling and soalrlng until equilibrium conditions wereestablished were, respectively, 130% and-f250%.

When the wood chips are accordingly subjected to a boiling and soakingtreatment with an aqueous solution of y an alkaline delignliyine' remagent, such as sodium hydroxide. lincrease in. weight is obtained whichsubstantially corre spends to the saturation value. However, it hns beenfound that the distribution of the chemise-l retained by the chips undersuch practice is for from uniform even after extended soaking perim ods,the greater proportion of reagent being presa ent in the outer portionsoi the chips while the centers, although saturated with liquid, containrelatively small amounts of ali or may be even entirely free of reagent.The inequality of trlbution of reagent between the exterior and interiorof the chips may be as great as lllzl or even greater.

We have discovered that if wood chips are saturated with a very dilutenon-delignifying so lution of an electrolyte, uniform distribution oi?lchemical reagent can be obtainedon subsequent impregnation withsolutions of alkaline dellgniiying chemicals such as sodium hydroxide orkraft liquor. Any aqueous, non-deligniying solution o1' any electrolytecan be used for the pretreatment, each electrolyte having an optimumrange of concentration for the purpose which can be readily determinedby the method to. be described below.

For instance, we have found that calcium chloride, sodium chloride,aluminum sulfate. sodium benzene sulfonate, and aluminum tarrate arevery satisfactory when used alone or as mixtures in the properconcentration. The action can be explained as probably due to the effectof the adsorption of the lons of the pretreatlng Experiments have shownthat presnondelignlfying electrolyte ou the laces with in the woodstructure which prevent the arp-- tion of the lons or the delignifylreagent, thereby minimizing the usual swelling of the wood and theformation ol electrical double layers oi' the ions of the alkalinereagent. e observed l'act that a specific optimum concentration existsfor each olierent electrolyte is an excellent indication that the edectis ed on electrolrinetlc phenomene..

The optimum concentration or range of concentration for pretreating woodwith any given none-deligniiying electrolyte may be determined by anyconvenient method. Theollowing method has been round quite satisfactorylor investieating the proper concentration of sodium chlorideelectrolyte ior use with black gum wood:

Blech. gum sapwood lolociis, l inch long (in the uber direction) by l.inch wide by inch thiclr, are saturated with l molar (5.845% by weight),lxlll-2 molar, l lll-4 molar, sodium chloride solutions by snbmergingand boiling the blocks in the res ectlve solutions for l. hour, thencooling to room temperature, and thereafter allowing there to sont, lorl. hour. The blocus are then drained, weighed to determine the absorbedmoisture and their volumes measured. They are then uniiormly impreunatedwith sodium hydroxide by Vlininersinlg them in a quantity ot sodiumhydrordde solution, so adiusted that the total caustic to wood ratio is@2li and the water to wood ratio is dll, und mmntaiuing the liquid andsubmerged wood at 9olllll fC. lor 2 hours. A control bloeit, which hasnot been pretreated with sodium chloride electrolyte, is treated underlilre conditions with sodium hydroxide. After cooling, the excesscaustic solution is removed, the voies of the blocks measured and all cithe blocus then out into nve equal sections (in the direction or thedrain). The weiehts ol caustic soda, water and wood are then determinedin each section. The data obtained are tabulated in Table l and is showngraphically in Figure #1.

Tanta li 4r J Grams sodium hydroade per ordm of wood (0. D. busto) P tyti Wood section ri? 89 Il'DOB 'r' .e I.. "o 50 u 011v h11 ll End mediateCenter mediate End Nono moet e029 Vloro 0.028 n.034 imolmNsci-.. .osv.o45 .oss .o45 .oso ixmeMNaol..- .l .on .osa .04s .04s .oec 55 ixio-ammol on .oss .een .034 .on

By referring to either Figure '#l or Table Ifit will be seen that 1 lllzmolar (0.0585% by weight) sodium chloride solution pretreatment resultedin the penetration oi thegreatest amount of caustic soda reagent intothe blocks and is therefore considered in the optimum concentrationrange. However, the penetration of thecaustic soda was not uniform withrespect to the cross section of the blocks and it was then necessary totreat wood' pretreated with 10-2 molar sodium chloride solution as abovedecribed for various lengths of time in the sodium hydroxide solution atthe ratios and temperatures above vdescribed in order to establish thelength of lov,r in Table II and are shown graphically in Figure #2.

vvhour treatment, but such treatment for 3 hours Thus, it may be seenthat by pretreating the blocks with -2 molar sodium chloride solution, a2 hour treatment in caustic soda solutions at 100 C. results in butlittle improvement in uniformity of distribution of the reagent over a 1Y results in substantially uniform distribution as evidenced by thenearly straight line concentration of the reagent through the crosssection of the block.

It should be noted that in determining the optimum concentration of theelectrolyte to be used by the method described above, the preciseoptimum concentration may not have been determined since sodium chlorideconcentrations slightly above or slightly below 1x10-2 molar might provesomewhat more efficient. However, it is considered that 1 102 molarconcentration lies in the optimum range of concentration for allpractical purposes.

Similar experimentatiomhas shown the optimum concentration of aluminumsulfate as a non-delignifying pretreating solution for black gum sapwood to be of the order of 1x10-5 molar, uniform inpregnation withsodium hydroxide then being achieved in the order of 31/2 hours.

Figure' #3 shows the distribution of caustic soda throughout the woodwhen a proper electrolyte pretreatment is given as compared withpretreatment with distilled Water alone before treating the wood at 100C. in caustic soda for 3 hours. It should be noted that the electrolytepretreatment results not only in a very uniform distribution of thecaustic reagent, but also in a higher total reagent on the wood contentthan either distilled water pretreatment or no pretreatment. It hasfurther been found that with no pretreatment, even 7 hours treatment at'100 C. with sodium hydroxide solution fails to give uniformdistribution of the caustic reagent in the wood.

Volume measurements of the blocks show that the wood swells in causticsolution Without pretreatment to as much as 27% greater than theoriginal volume of the wood, while with electrolyte pretreatment theincrease is only' 14%. This reduction in swelling is considered animportant aid to uniform penetration.

In practice, the time required for treating the wood chips withdelignifying solutions at below 100 C.is muchless than that describedabove with the use of blocks 1 inch by 1 inch by inch. It is a wellestablished fact that penetration proceeds much 'more rapidly with thegrain of wood than at right angles thereto. Commercial wood chipsusually range from to 3A inch in length. Consequently, they are muchmore rapidly penetrated than the blocks used in illustrating theoperation of the principles of our invention. For example. a. soakingtime of chips pre-impregnated with a non-delignifying electrolytesolution with a caustic soda solution of 30-60 minutes is usually foundample to effect the uniform distribution of reagent.

After the wood chips have thus been saturated with a solution of anon-delignifying electrolyte according to the method described, and ifthe avoidance of undue dilution of subsequent delignifying solutions isdesired, removal of the residual non-dellgnifying liquid in which theboiling and soaking has taken place may be accomplished by drainage,preferably in the presence of steam at a somewhat higher temperaturethan that employed during the air removal treatment. This permits thedisplacement of excess surface liquid from the chips and may also serveto boil off some of the liquid held internally in the wood. However,under these conditions care must be taken to avoid any exposure of thechips to air.

To the saturated, drained chips a delignifying solution such as, forexample, sodium hydroxide, may then be added at a temperature preferablyjust under its boiling point. In calculating the concentration of thedelignifying reagent in such solutions, account should be taken of theamount of water held by the saturated chips. 'I'his will vary over aconsiderable range such as, for example, 100% or less of the weight ofthe Wood to 250% or more, depending on the species of wood beingtreated. It will also v ary with the proportions of different kinds ofgrowth of the tree such as sap wood, heart wood, springwood, andsummerwood. The chips are thereafter allowed to soak in a delignifyingsolution for about 1 hour or less while the temperature is preferablymaintained close to 100 C. to provide the best diffusion conditionsWithout substantially exceeding a temperature of 100 C.

'I'he Wood. after being uniformly penetrated with alkaline cookingliquors by this process, is then capable of being delignified moreeconomically to a more homogeneous pulp than is possible by processes inwhich uniform association of the alkaline reagents prior to providingreactive temperatures is not attained. The present process can bereadily carried out in the equipment in general use in the pulpingindustry with the important advantages of the use of milder digestingconditions and a smaller requirement of delignifying chemicals.

The reaction between sodium hydroxide, for example, and the ligneouscomponent of the wood at temperatures in the range of 150-175 C. isquite rapid, so that when the lignin demand of reagent has beenuniformly distributed throughout the woody structure in accordance withthe provisions of our invention, the required reac-v tion not onlyproceeds uniformly but quite rapidly. As a result, the time forcompleting the reaction may be very much reduced with consequent minimumamount 'of degradation to the cellulose itself. A further advantageaccruing from the discoveries of the present invention is the use of alower delignifying temperature with consequent milder conditions towhich the cellulose itself is subjected. Both shorter time of digestionand lower temperatures may be employed with marked beneficial effectover prior practices.

The pulp derived from the modified cooking schedules, made possible bythe uniform association of the proper amount of reagent with the woodprior to the attainment of pulping conditions, is not only animprovement over alkaline pulps by prior processes with respect to homo-Seneity and substantial freedom from degradacasacca t I tion,vbut theyield is higher by usually 3%5% than the yields ordinarily resultingfrom the best conventional practices.

The following are examples of the application of the principles of thepresent invention to the pulping of yellow birch chips with a sodium hy-.droxide solution carried out in standard pulp mill equipment. It mustbe understood, however. that the invention is not limited to thereagent, species of wood, nor specic conditions set forth in theseexamples.

ExnceLlA Yellow birch chips, preferably free of dirt and knots, arecharged into a conventional digester and covered with a dilute,non-delignifying electrolyte solution at optimum concentration, forexample, 1x10-il molar sodium chloride or 1 10s molar aluminum sulfate.Steam is admitted through the distributing mains provided for thatpurpose near the bottom of the digester, and the solution is boiled for30 minutes under freely vented conditions to Yaccomplish the removal ofair from the chips. During this boiling all of the chips should be keptunder the surface of the liquid.

An alternate practice is to supply steam to the sealed digester until atemperature slightly in excess of 100 C. is attained and then to relieveair and steam from the top of the digester intermittently over a. 30minute period. Upon the completion of the boiling treatment, thejsteamrelief valves are closed and a 30 minute period is allowed for thesaturation of the chips with the dilute sodium chloride solution. Duringthis time steam under pressure may be admitted above the liquor level tofacilitate the impregnation. i

The excess solution is then drained from the digester and may be usedfor the treatment of subsequent batchesof chips. In the case ofpretreating highly resinous woods, the resins tend to iloat up to theliquid surface during the boiling period and form a layer on the surfacewhich is very sticky and tends to adhere to the walls of the digester,the wood, or anything else with which. it comes in contact. This layerof resins may be removed by any convenient means, such as drawing offthe upper layer of the liquid above the chips before draining the massof electrolyte solution from the digester.

Caustic soda solution at a temperature of about 70100 C., but preferablynot exceeding 100 C., is then added to the digester at a concentrationand in an amount which is` adjusted tobring the proportion of wood (ovendry basis): caustic soda: water equal to 100:18z-i00. For example, 293pounds of 8.5% sodium hydroxide solution will be required for each 100pounds of wood if a 130% increase in weight on the original dry weightof wood has occurred during theA boiling and soaking treatment with thedilute, non-delignifying electrolyte solution heretofore described. Thewood is allowed to soak in the cooking liquor for 30-60 minutes,preferably at not more than 100 C. The digester is then sealed and steamis added to raise the tempera-` ture to 170 C. as rapidly as possible.Digestion is continued at 170 C. for about 2 hours, and the digestercontents then discharged into a blow pit where it is thoroughly washed.A yield of 53% of screened pulp is thus obtained, having a single-stagebleachability requirement of about 26% bleaching powder, containing 35%active chlorine, as determined by the permanganate Examen: B

Yellow birch chips, preferably free from dirt l and knots, are chargedinto a digester and covered with an optimum concentration of a dilute,

. non-delignifying electrolyte solution such as, for

example, 1x10-2 molar sodium chloride or 1x10-5 molar aluminum sulfate.The solution l5 is brought tolthe boiling point by direct steam andboiled for 30 minutes under freely vented conditions to accomplish theremoval of air from the chips. vOn the completion of the-boiling period,the heating is discontinued and the chips allowed to soak for aboutminutes in order Vto saturate them with the electrolyte solution. Duringthe boiling and soaking, all of the chips should be kept under thesurface of the liquid.

The alternate practice described in Example A of sealing the digesterand supplying steam until the temperature of the mass of chips andelectrolyte solution slightly exceeds 100 C. and then to relieve the airand steam from the top of the digester intermittently over a :iO-minuteperiod may be practiced if desired.

The excess solution is then drained from the chips as in Example A and310 pounds of 12.8% sodium hydroxide solution for eachl 100 pounds ofwood (oven dry basis) are then added to the pretreated chips. The chipsare allowed to soak in this caustic soda solution for 30-60 minutes at atemperature of 95100 C. The excess solution is then drained from thechips and may be utilized for impregnating subsequent batches of chipswith delignifying solution. 60%-65% of the weight of caustic sodasolution originally added can be recovered in this manner.

The treatment described leaves each chip in the batch thoroughly anduniformly impregnated with the caustic soda reagent in the ratio of18-19 parts of NaOH for each 100 parts of wood (oven dry basis). Thechips are therefore in an ideal condition for subsequent delignicationwhen heated to reactive temperatures, especially in continuous digestionequipment, since they are impregnated with suilicient delignifyingreagent to insure complete pulping without the necessity of subsequentdiiusion of reactive chemical from the usual pool of surrounding liquor,as is common practice in conventional processes.

Furthermore, when subjected to reactive tem-- peratures, the chips arepulped very much more rapidly than in conventional processes, forexample, the impregnated and drained chips, as described above, may beheated to 170 C. in 15 minutes 'and maintained at that temperature for25 'minutes or even less. At the end of this treating period a yield of52%53% of screened pulp having a bleach value of 20%-25% may beobtained.

Consideration of the theoretical aspects of the present invention andthe reason it oii'ers the very considerable advantages that it doesinvolves, iirst, an understanding of a. The removal of air from smallcapillaries, and

b. Certain aspects ofthe direct impregnation of wood with causticdelignifying solutions as practiced in prior processes.

lso

By direct impregnation is meant the treatment of wood wi-th a caus-ticdelignifying solution without using the pretreatment step with anon-delignifying solution as described in connecy tion with thisinvention.

a. Removal of air from small capillaries Since the structure of wood isknown to involve the presence of not only microscopic pores andcapillaries, but also submicroscopic pores, the problem of the retentionof air in very small cavities is one of fundamental importance. The airis held in these cavities with great tenacity and its displacement isexceedingly difilcult because of the minute dimensions which prevent theeasy flow of liquid and gas through these pores. We have found` that theuse of strong suction or various low pressure devices does not eectivelyremove the adsorbed air. Therefore, in practice we depend upon thepreferential wetting of the 'surfaces of the Wood cavities by water orother treating liquids to thereby displace the adsorbed air lm.

The nature of the problem of the removal of air from the vpores andcapillaries of wood is illustrated by reference to Figure #4 showing aset of three small test tubes, 35, 36 and 31. These tubes are invertedand placed in water to varying depths, one of the tubes being completelysubmerged. When the water is heated, the air inside the tubes willexpand and bubbles of air will be noticed leaving the tubes. The amountof air which can be removed in this way will depend on the temperatureof the bath. As the temperature increases, Ithe vapor pressure of thewater increases. Obviously the pressure of the gases within the tube isthe sum of the partial pressure of the air and water vapor at a giventemperature. Hence, as the temperature increases, the eiect of the watervapor upon the total vapor pressure of the system becomes more and moresignificant, and at boiling temperature the pressure of the waterbecomes equal to the pressure at the mouth of the tube. In each bubblewhich leaves the tube there will therefore be a portion of air and aportion of water-vapor.

Since the amount of air is limited by that quantity which originallyfilled the tube, and also since there is an unlimited amount of watervapor obtainable (from the supply of liquid water present in the tube),there will be a constant tendency to remove air from the tube,displacing same with water vapor. This displacement will occur as longas any air is presenft because the pressure within the tube will alwaysbe greater than the atmospheric pressure at the mouth of the tube at thetime of boiling by an amount that is equal to the partial pressurewithin the tube. The development of water vapor at a pressurecorresponding to the temperature of the bath provides a means thereforeof flushing the air completely from the tube. 1f it is desired to havethe tube lled with water in the liquid phase, it is only necessary toeither cool the system suiiiciently to allow the water vapor within thetube v moved; whereas the tubes 36 and 31, which partially extend abovethe surface of the liquid, lose their air content more slowly. In fact,the exposed tubes do not become completely filled with water even afterprotracted boiling.

Figure #5 illustrates one form of this experiment in which a closedglass tube partially Iilled with water and with air above the waterlevel is employed. Immersed in the water within the tube is a glass rod,38, to which are attached some capill-ary tubes, 39 and 40; all of whichare closed at one end. Half of the tubes 39 are inverted; the other half40 having their open ends pointing upward toward the air space. Evenafter repeated heating to temperatures appreciably above 100 C. followedby intermediate coolings, air fails to leave these tubes. Thus. eventhough the open ends of these tubes would appear to oiier an opportunityfor the free escape of air through the water to the space above,actually the capillaries contain most of their original air contentbecause the pressure in the closed space above the Water increases atapproximately the same rate as that inside lthe rtubes.

Wood chips comprise a network of similar microscopic and submicroscopiccapillary tubes largely lled with air which it is desired to remove.From the foregoing description of the experiments illustrated in Figures#4 and #5, it is clear that the air in the capillaries and pores of thewood can not-leave the chips as long as the digester is sealed. It isonly by providing adequate venting under such conditions as will permitthe very large volume of air entrained inthe wood to escape that thepores of the chips can be filled with liquid. The conditions employed incommercial practice of relief in the digestion of wood do not provide anvopportunity for this complete removal of air during that period of thecycle prior to the attainment of active digesting temperatures when itis most important that the delignifying liquids shall gain free accessto the innermost portions of the wood.

Partial air removal leaves residual air entrapped in the innermostinterstices of each chip. Upon providing elevated temperatures fordigestion, even a relatively small volume of entrapped residual air willprevent the necessary contact between 4the treating solution and asubstantial l portion of the inner structure of each chip. Thisinevitably results in non-uniformity of distribution of the treatinguid. Therefore, the importance of obtaining substantially complete airremoval can be readily appreciated.

d. Direct impregnation Swelling.-Since wood is known to increase to amaximum volume of approximately 20% during immersion in hot water for-20 minutes, it would seem that swelling of the carbohydratelignincomplex occurs very rapidly. This maximum swelling can even occur whilethe interior of the wood subdivision is substantially free of moisture.When alkalies are present the maximum swelling within the time cyclementioned is somewhat greater than when water alone is used. The failureof the alkali to uniformly penetrate the wood is at least in partattributable to the partial or complete closure of the microscopic andsubmicroscopic pores in the wood by the swelling of the material aroundthese pores.

After the removal of the lignin by the treating liquors, the wood berspossess the property of absorbing and binding relatively largequantities of water. This action results in an enormous swelling of thefibers; therefore, unless the wood subdivision is thoroughly impregnatedwith the delignifylng solution before the treating process is started,the ilbers residing in the'outermost portions of the subdivision 'willbe at least partially deligniiied, absorb water, and become more highlyswollen. This condition will tend to further constrict or may even closethe pores and capillaries, making it extremely diicult for thedelignifying solution to penetrate further into the wood.

Absorption-The outermost surfaces of a wood subdivision absorbconsiderable quantities of chemicals from the treating solutions,particularly those of an alkaline nature, so that any solution which maypenetrate the subdivision is depleted with respect to the concentrationof the delignifying reagent. The significance of this depletion oi'reagent from the solutions by absorption is apparent when it isconsidered that absorption is a surface phenomenon and that the surfacesci a gram of wood have been estimated to be about llwoll squarecentimeters.

Electrical double layer formation-When wood is treated with solutions orclelignifying chemicals, ions are absorbed on the surfaces oi thecapillary Walls with the formation of electrical double layers. Theseelectrical double layers may auect the penetration oi cooking liquorsinto the wood in several ways.' First, the formation or such doublelayers, even though their thickness docs not exceed several moleculardiameters, may stili be sumclent to seriously decrease the effectivecross-sectional areas of the very minute capillaries of the wood sincethe rate of how of liquids through small capillaries is proportional tothe fourth power of the radius; second, the movement of liquids in thecapillaries past the electrical double layers on the walls produces astreaming potential which always has the edect oi retarding the ow ofsolutions through the capillaries. ln a similar manner the ow ofsolutions through the membrane produces a membrane potential having asimilar retarding eii'ect.

habla of permeabilitylt is possible that at least some oi the membranesin the wood may be impermeable or only semiperrneable to the delignifying chemicals contained in the cooking liquors; consequently,deligniiying ions in solution, or at least some of them, can notpenetrate beyond these impermeable membranes.

Mechanical blocking of membranes-The action of delignifying reagents mayproduce colloidal ysubstances which mechanically block or plug theminute capillaries of the wood.

ChemicalA action- When wood is impregnated with delignifying solutions,a chemical reaction takes place between the reagents and variouscomponents of the wood. This reaction, of course, occurs first in theoutermost portions of the wood subdivision so that the impregnatingliquor, as it penetrates into the wood, is gradually depleted withrespect to the concentration of the reactive chemicals in the liquor.

In the present invention, as has been described, after pretreatment witha non-delignifying electrolyte under proper conditions, a. substantiallyuniform penetration of the delignlfying solution into the wood issecured. As has been indicated, the concentration` of the effectivedelignifying chemical is substantially uniform throughout the individualpieces of wood. 'I'he effect obtained with a. dilute solution ofnon-delignifying electrolyte may be explained as being due to thecombined influence of a number of factors which are described in theLfollowing paragraphs:

By employing the proper concentration of the stances such as proteins,etc. which are usually in colloidal solution is that pH or hydrogen ionconcentration at which the substance apparently changes from acidic to abasic character or vice versa. This is primarily due to a surfaceadsorption of hydrogen ions. However, inert substances such as quartz,glass, cellulose, etc. are also capable of absorbing ions on theirsurfaces with the formation of electrical double layers. The statisticaldierence between the numbers of positive. and negative charges presentin this electrical double layer formation is lrnown as the zetapotential'. rit the iso-electric point the number of, positive andnegative charges present in the electrical double layer is exactly equaland the zeta potential is consequently aero.' With changingconcentrations oi electrolyte, the `net charge on the surface changessign as the isoelectric point is passed; consequently the isoelectricpoint is that point at which the net surface charge is just ori/theborder or" reversing its sign. If the electrolyte adsorbed is an acid,base or an acidic or basic salt, the iso-electric point occurs at adenite hydrogen ion concentration (pH) and is donned as such. However,neutral salts maybe readily adsorbed and in such cases the iso-electricpoint occurs at a denite' concentration of the salt, usually a verydilute concentration.` The iso-electric point is cornmonly dened interms of the molar concentration of the electrolyte solution from whichthe adsorption takes place.

Pertinent phenomena occurring at the isoelectric point are zero zetapotential and minimum or no swelling of the wood. As a result of thezero zeta potential. the resistance to the flow of liouids due tostreaming and membrane potentials is reduced to a minimum. Since theeffect of swelling is to reduce the cross-sectional area of capillariesin the wood through which the liquid must pass, aA decrease in theamount of swelling results in improved penetration.

The adsorption oi caustic soda or other delignifyine reagents on thesurfaces of the pores and capillaries of the wood is also reduced orprevented by pretreatment with non-delgnifying electrolytes. If theadsorptive powers of the wood are satised by some electrolyte prior tothe addition of the delignifying reagent, the

concentration of such delignifying solution is not gradually decreaseddue to adsorption of the reagent from the solution in its passagethrough the pores and capillaries of the wood.

The treatment of the wood with non-delignifying electrolyte solutions iseffective in obtaining more uniform distribution of the subsequentlyadded chemicals because this treatment is given prior to the addition ofthedelignifying reagents. The surfaces of the pores and capillaries ofthe wood may be conceived to be given a protective coating of adsorbedions before the addition of the delignifying reagents.

This "protective coating, however, has no effect on the reactionsbetween the subsequently added delfgnifying reagents and the material,l

such as lignin, present in the wood when reactive stable and are onlydisplacedrather slowly by the addition of a solution of relatively highconcentration of delignifying reagent, such displacement does, ofcourse, ultimately take place. But, according to the present process,before this displacement has occurred, the wood has been uniformlyimpregnated with the desired amounts of delignifying reagents and nofurther movement of delignifying reagents from the solution .surroundingthe wood into the interior of the subdivisions is necessary. The mereaddition of the same concentration of non-delignifying electrolyte tothe solution of delignifying reagents vwould be wholly ine'ec'tive sincein that case a mixture of thenon-delignii'ying electrolyte anddelignifying reagent ions, with the latter predominating, would beadsorbed. Furthermore, the concentration oi ions on the surfaces of thewood would be many times that required to produce iso-electricconditions.

The preconditioning of the surfaces of the n Wood with very dilutenon-delignifying electrolyte solutions when applied to the manufactureof wood pulp permits the production of substantially higher yields offar more uniform, pulps than is possible by any of the well knowndelignifying systems..

The improvement over'prior practices is especially marked when alkalinetreating solutions are employed. Another distinguishing feature of thepulp resulting from the use of the present invention is the reduceddepolymerization of the cellulose reflected in a much highercuprammonium viscosity as distinguished from the best commercial sodaand kraft products. The residual impurities in the digested pulpresulting from this invention are quite uniformly associated with thecellulose iibers.` In other words, the pulps of the present inventionare. entirely homogeneous, This condition permits a very economicalbleaching of the pulps to -any desired standard of whiteness. Forexample, a pulp with a bleachability of 40% may be brought to thestandard ofA whiteness ordinarily required for magazine or book paper bya carefully controlled chlorination employing chlorine gas, causticizingthe chlorinated pulp, washing, and then treating with 2%`5% calciumhypochlorite with a reduction in pulp yield of not to exceed the orderof 2.5%. Over-all yields of bleached pulp from 50%-52% are readilyattained, which represents an increase of 20%-25% of actual bleachedpulp over standard mill practices of the order of 41%.

A characteristic feature of pulp produced in accordance with the presentinvention is the freedom of said pulps from the products of diegrationof the cellulose bers which have been excessively treated withdelignifying liquors. has been indicated before that in connection withthe ordinary digesting processes, the cellulose fibers from theoutermost portions of the Wood chips are severelyl degraded from theirprolonged contact after deligniiication with the surrounding pool ofdelignifying liquors. In the present invention, by distinction, thedcligniiication proceeds uniformly, simultaneously, and quickly througheach subdivision of the entire mass of wood chips. Consequently, everypart Itl of each fiber is delignied at any given time to substantiallythe same extent. Thus the deligniflcation conditions may be terminatedVat any desireddegree of lignin removal, with the resulting pulp productcharacterized by the uniform distribution of its celluloslc andnon-celluloslc components. Because the treating solutions have a strongpreferential action on the ligneous -material, under these digestingconditions the cellulose iibers are protected and remain substantiallyundegraded.

It is obvious from the foregoing that the process of the invention maybe used to practical and economical advantage in impregnating woodymaterials with other than digesting solutions. For example, solutions oidyestuifs, fireproofing compounds and other liquids may likewise beuniformly distributed through Wood or woody materials. l

As many changes could be made in carrying out the above compositions andprocesses without departing from the scope of the invention, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

We claim:

l. A process for the delignification of wood comprising treating thewood in the form of small pieces with a heated water solution of anon-delignifying salt 4forming an electrolyte in the solution of acencentration low enough to be just suflicient to deposit by 'adsorptionon the surfaces of the capillaries and membranes ions in layer formationproviding protective coatings on said surfaces and facilitating thepassage of an alkaline delignifying agent to the interiors of the woodpieces, subjecting said wood pieces to said electrolyte solution undertime and temperature conditions effecting said adsorption of ions fromthe electrolyte on the interior surfaces of' the wood, while avoidingdelignication and maintaining the accessibility of the membranesV andcapillaries, then subjecting said wood pieces so impregnated with anelectrolyte to soaking in an alkaline digesting liquor at a temperaturebelow the digestion pointv and for a time sutilciently long to effect acomplete impregnation of the wood pieces with the liquor, and thenraising the temperature to digest the wood. 2. The process as set forthin claim 1 in which the electrolyte solution is at the iso-electricconcentration for the wood so that the numbers of positive and negativecharges present at the surface layers are substantially in equilibrium.

3. The process as set forth in claim 1 in which the wood is uniformlyimpregnated with a delignifying reagent at a concentration whichprovides an amount of such reagent in the quantity of liquor adsorbed bythe woodsufilcient to complete the dellgnification of the wood forpulping. 4. The process as set forth in claim 1 in which the electrolytesolution comprises a solution of sodium chloride of -2 molarconcentration.

5. The process as set forth in claim 1 `in which the electrolytesolution comprises a solution of r aluminum sulfate of 10-5 molarconcentration. SOLDON FELDMAN. FREDRICH OLSEN.

CERTIFICATE 0E CORRECTION. Patent No. 2,228,5LL9. January 1li, 19in.

soLDoN RELDMAN, ET AL.

It is hereby certified that error appears in the printed specificationothe above numbered patent requiring correction as follows: Page5,*fi'rst column, line 'Il-72, for "tarrate" read '-tartrate-; page 6,second column, line 51p, for "d. Direct impregnation" read -b. Directimpregnation-g page 7, first column, line 9, for "Absorption" read--Adsorption--g line lO,

for "absorb" read --adsorb; lines 16 and 17, for "absorption" read"adsorption", line 25, for "absorbed" read ads rbed; same page, secondcolumn, line 5, for "absorption" read --adsorption; line l5, for"absorbing" read --adsorbing; page 8, first column, line 56-57, for"degration" read degradation-4; and that the said Letters Patent shouldbe read with this correction therein that the same may conform to therecord of t1' e case in thev Patent Office.

signed and sealed this 15th day of May, A. D. 19in.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

